1. Field of the Invention
The present invention relates to the catalytic hydrogenation of aldehydes to alcohols. The present invention specifically pertains to an improved catalyst and a catalytic process for hydrogenating aldehydes to alcohols with a significant improvement in selectivity evidenced by a reduction in the production of undesired by-products.
2. Description of Related Art
Hydrogenation of aldehydes to produce alcohols has long been practiced. The reaction of an aldehyde with hydrogen generally is carried out in the presence of certain reduced metal compounds which act as hydrogenation catalysts. The commonly used commercial hydrogenation catalysts include copper chromite; cobalt compounds; nickel; nickel compounds which may contain small amounts of chromium or other promoters; mixtures of copper and nickel and/or chromium; and a mixture of reduced copper oxide-zinc oxide (i.e., copper-zinc oxide).
Not surprisingly, all of these hydrogenation catalysts are associated with one or more disadvantages when used for commercially hydrogenating aldehydes to alcohols. Most of these catalysts such as copper chromite, the cobalt compounds, the nickel catalysts and the reduced copper oxide-zinc oxide catalysts, exhibit a less than desired selectivity. Stated otherwise, when hydrogenating aldehydes using such catalysts, the quantity of by-products formed may be higher than desired. Such by-products reduce the desired selectivity of the aldehyde to alcohol conversion and generally must be removed from the hydrogenation product prior to subsequent use of the alcohol. See European Patent Publications Nos. 0 008 767 and 0 074 193. Furthermore, copper chromite catalysts are difficult to prepare and have serious toxicity problems associated with their use, and the cobalt compounds are significantly more costly.
When using nickel catalysts, the principal by-products are ethers and hydrocarbons (paraffins); while the use of reduced copper oxide-zinc oxide catalysts yields esters as the principal by-product. The amount of by-products formed may be anywhere from about 0.5 to about 3.0 weight percent and even higher, based on the total weight of the reaction product.
For example, in the catalytic hydrogenation of butyraldehyde to butanol over a nickel catalyst, a small amount of butyl ether forms; while using a reduced copper oxide-zinc oxide catalyst for the same reaction yields by-product n-butyl butyrate in minor amounts. The ethers form azeotropes with the alcohol hydrogenation products and water frequently present in the product from the feed streams. Thus, a substantial amount of energy is required to separate by-product ethers from alcohols and significant losses of alcohol normally are encountered. For example, separation of butyl ether from butanol required for butanol to pass purity specifications, such as the specification for making acrylates, requires a series of costly distillation steps and because of the butyl ether-butanol azeotrope, four pounds of butanol are lost for every pound of butyl ether formed. Such losses may render the use of an otherwise advantageous hydrogenation catalyst, commercially unattractive.
While by-product esters may be easier to remove, the separation costs and associated losses are not inconsequential. Ester formation leads to a loss of alcohols via the ester stream purged from the bottom of the alcohol refining still in a typical recovery process. The approach used in European Patent Publication No. 0 074 193 to avoid this loss, which involves the recovery and concentration of the esters and then their conversion to additional alcohol by hydrogenolysis in another reactor containing reduced copper oxide-zinc oxide catalyst, requires additional equipment. Furthermore, the amount of esters formed generally increases within increasing temperature in the catalytic hydrogenation reactor. Thus, to minimize by-product ester formation when using reduced copper oxide-zinc oxide catalysts, hydrogenation processes may need to be operated at relatively low temperatures. This is particularly true when an ester such as propyl propionate is the by-product because of the difficulty of separating such esters from the desired alcohol using ordinary distillation techniques. Unfortunately, operation at lower temperatures results in a reduced rate of catalytic hydrogenation.
The tendency of the reduced copper oxide-zinc oxide catalysts to yield higher levels of esters at higher reaction temperatures also complicates the implementation of conventional catalytic techniques. Normally, to compensate for the gradual and unavoidable loss in hydrogenation catalytic activity with time, it is conventional practice to increase reaction temperature with time. When using reduced copper oxide-zinc oxide catalysts, however, such temperature increases lead to an increased formation of ester by-products, thus further complicating subsequent product purification procedures or if the level of by-product ester formation increases above tolerable limits, necessitating an earlier change in the catalyst charge than dictated by hydrogenation rates.
The need to operate the reaction at lower temperatures also complicates the process by requiring either more costly reactors or an increase in the number of adiabatic reaction stages with intercoolers. Furthermore, less useful energy is recovered from the heat of reaction at lower temperatures.
As is evident from the foregoing, a need exists in the art of catalytic hydrogenation of aldehydes to alcohols for a catalyst having improved product selectivity, particularly a catalyst which retains its high selectivity at the high temperatures needed to maximize reaction rates and energy efficiency.